FIG. 1 shows the structure of a conventional microcolumn, a miniaturized electron column. The electron column includes an electron emitter 1 for emitting electrons, a source lens 3 formed of three electrode layers, that is, an extractor electrode layer 3a, an accelerator electrode layer 3b and a limiting aperture electrode layer 3c, and configured to cause electrons to be emitted from the electron emitter and to effectively form the emitted electrons into an beam, a deflector 4 for deflecting the electron beam, and a focus lens 6 for focusing the electron beam on a specimen. Although the construction of the lenses and the deflector may be modified in various ways as needed, electron columns having the above-described construction are generally used.
In the electron column having the above-described construction, a negative voltage is applied to the electron emitter 1. All of the electrode layers of the source lens 3 may be grounded, or a positive voltage may be applied to the extractor electrode layer 3a, so that electrons can be smoothly emitted from the electron emitter 1. Furthermore, a large negative voltage may be applied to the electron emitter to increase electron beam energy. Furthermore, a negative voltage may be applied to the extractor electrode layer to appropriately maintain the difference in voltage between the electron emitter 1 and the extractor electrode layer 3a. The electrons are emitted according to an electric field formed by the difference between the voltage that is applied to the electron emitter 1 and the voltage that is applied to the extractor electrode layer 3a. The emitted electrons are formed in a manner similar to beam intensity distribution (a), and pass through the accelerator electrode layer 3b and the limiting aperture electrode layer 3c, thus being effectively formed into an electron beam. Generally, the accelerator electrode layer and the limiting aperture electrode are used in a ground state.
The electron beam formed by the source lens 3 is deflected by the deflector 4 and is then focused on a specimen. In the electron column, an Einzel lens is used as a representative focus lens 6. The Einzel lens has a structure in which three electrode layers E1, E2 and E3 are layered.
Furthermore, the last electrode layer of the lens is generally used in a ground state, so that the energy of an electron beam that reaches the specimen is mainly determined by the voltage applied to the electron emitter. That is, the difference between the voltage applied to the electron emitter and the voltage applied to the last electrode layer of the lens (the last electrode layer of the focusing lens) critically affects the electron beam energy of the electron column.
In the elements of the above-described electron column, tungsten is mainly used as material for the electron emitter 1, and a tip, having a pointed end and a radius of several tens of □, is used to obtain the electron beam. Furthermore, a long cylindrical tip is used not only to obtain a stable electron beam, but also to increase the lifespan of the tip. Furthermore, in the microcolumn, the lenses are manufactured through a Micro Electro Mechanical System (MEMS) process.
The spot size, that is, the probe beam size, of an electron beam, which is focused by the electron column that generates the electron beam, is a very important factor in the performance of the corresponding column. The spot size of an electron beam focused on a sample by the electron column is an important factor that determines the resolution of the electron beam in a typical electron microscope, or determines the line width of a pattern, which is formed by the electron beam, in an electron beam lithography.
FIG. 1 shows a diagram of elebron optics by a miniaturized electron column. As shown in FIG. 1, the intensity of the electron beam emitted from the tip is represented by a Gaussian distribution, and the electron beam is broadened at a slightly divergent angle of αe. In this case, only electrons, which are included in the slightly divergent angle, pass through a limiting aperture so as to reduce the diameter of a probe beam and optimally control the beam. α0 is defined as an effective divergent angle. The reason for this is because most of the electrons that have passed through the limiting aperture reach the specimen. The current that flows through the limiting aperture is about 1/10000 of the current emitted from the tip. The reason for this is because the radius of the limiting aperture is very small, about several micrometers. The electrons that have passed through the limiting aperture have electron-electron scattering while passing through a narrow space, and thus the energy broadening of the electron beam occurs.
The electrons that have passed through the limiting aperture pass through the deflector, in which a deflection-aberration occurs. Since such a deflection-aberration is relatively unimportant compared to other aberrations, it is frequently excluded from calculations. The electrons finally pass through the Einzel lens. The Einzel lens functions as a convex lens that converges light in an optics system. Accordingly, the electron beam that has passed through the lens reaches the sample within the range of a slightly divergent angle of α1. In the process of convergence of the electron beam by the lens, a chromatic aberration and a spherical aberration occur as in the optics system, and a coma is created because the electrostatic lenses are not arranged parallel to each other.
Accordingly, minimizing the chromatic aberration and minimizing the diameter of the probe beam depending on the contraction of each of the lenses are the major factors in the design of the electron column.
The diameter of the probe beam must be minimized to perform a precise process using an electron beam. However, there are limitations in the ability to reduce the diameter of the probe beam due to various factors. The greatest of the various factors is aberrations, and other factors include electron-electron scattering between electrons, distortion caused by the deflector, diffractions and the like. Such aberrations are classified into a chromatic aberration, a spherical aberration and a coma. The chromatic aberration and the spherical aberration are generated by each lens, which is one major problem that must be solved to improve the characteristics of the probe beam.
The lenses of the electron column are important in relation to the above-described characteristics of the probe beam. The electrostatic lens affects the moving trajectory of the electrons in the same manner as an optics lens that affects the path of light.
FIG. 2(a) shows the state in which light is converged by an optics convex lens, and FIG. 2(b) shows the state in which an electron beam is converged by the electrostatic lens. In the case of the optics lens, the light is refracted or converged by passing through media having different refractive indices. In the case of the electrostatic lens, the electron beam is refracted by the potential difference generated by the same medium. The optics lens is made of a single material having a constant refractive index and keeps the velocity of the light constant. In contrast, the electrostatic lens causes continuous variation in velocity of the electrons while the electrons pass through the lens because the curvature of the equipotential surface thereof varies.
Generally, the electrostatic lens includes two or more circular electrode plates, and operates in such a way to form an electric field between the electrode plates by applying voltages to the electrodes and to control the movement of an electron beam. In particular, as shown in FIG. 2(b), in case the electrostatic lens include three electrodes and is designed such that the energy of the electrons entering the lens and the energy of the electrons passing through the lens are kept constantly by applying the same voltage to two end electrodes, it is called an Einzel lens. When the voltage applied to the two end electrodes of the Einzel lens is V1, and the voltage applied to the central electrode of the Einzel lens is V2, the lens enters into retarding mode if V1>V2, and the lens enters into accelerating mode if V1<V2.
The focus lens is important in relation to the spot size of the electron beam emitted from the electrostatic lens of the above-described electron column to the specimen.
Generally, in the electron column, focusing is performed using the focus lens, such as the Einzel lens. The retarding mode or the accelerating mode is mainly used to perform focusing when the Einzel lens is used in a electron column. In the Einzel lens, the upper and lower electrode layers E1 and E3 thereof are grounded and a voltage is applied only to the intermediate electrode layer E2, and thus the Einzel lens enters into the retarding mode or the accelerating mode. Accordingly, in the retarding mode, a negative voltage, which is lower than that applied to the upper and lower electrode layers E1 and E3, is applied to the intermediate electrode layer E2. In the accelerating mode, a positive voltage, which is higher than that of the upper and lower electrode layers E1 and E3, is applied to the intermediate electrode layer E2.
As described above, in the conventional focusing method in the electron column, the upper and lower electrode layers E1 and E3 are grounded and a necessary voltage is applied to the intermediate electrode layer E2, and thus the operation of the electron column is convenient. However, the electron beam that reaches the specimen has a large spot size, so that the method is disadvantageous in that it is difficult to use it to perform a high resolution patterning process, or to use it in an electron beam lithography.